I'm hoping someone can answer, as I cannot. With power assisted cable, you can turn the wheel from side to side and move the outdrive (albeit with a good deal of resistance) even if the engine is off. Can you do the same with straight hydraulic? I assume you would need the pump working to do so.

Embarrassing, as I have hydraulic steering on my current boat, but never tried it.

A true hydraulic system does not require any power so yes you can operating the steering without the engine being on. Based on the manual the OP provided this will still function as the Auto pump is in parallel to the helm pump.

I assume that it's cable-driven, but it looks like a weird system. There's one hard line coming off the starboard side heading back up to the helm. Since it's only one line I can only assume it's cable. However, right next to the cable inlet on the steering rack, there's two hard-crimp fluid lines that scream high-pressure hydraulic fluid, but then they T into two very large soft lines on the back of the motor, and I don't know what runs through that.

So, in any event, looks like it's cable steering (though I'm still curious what those lines are -- part of the power assist somehow?..)

I've talked to the Octopus drive guys and it looks like it'll be around 1000$ for the octopus drive for the cable steering, plus around 1300$ for the Garmin ECU/CCU to power/control the drive. Not too bad, but I was hoping for cheaper. That project will have to wait until the summer, I think. Dropping 10k into the boat already is leaving me a bit sore.

I'm away on another trip through Monday, and can post a picture when I get back. The lines look like coolant lines (they're ~1 inch diameter soft lines running side to side across the back of the motor), but I haven't tried digging into anything there. It's definitely not connected to the trim tabs, that has a separate system.

It’s Mercruiser’s fuel management system to reclaim the return fuel from the fuel rail that feeds the injectors to recirculate the fuel with new fuel drawn from the gas tank. On cars this would get returned to the fuel tank but not on boats of this vintage. The system required cooling so they plumbed engine coolant to it.

The hard lines heading to the right in the picture there are the ones I'm talking about. One goes into that side-to-side plastic tube connected to large soft 90 degree bends that go under to the sides of the motor and seem a lot like coolant lines. The other heads straight down and I didn't bother to see how far it went.